This invention relates to a process for the pretreatment of a metal workpiece for painting, the surface to be painted being cleaned with a solution containing degreasing agents and being coated with a corrosion-inhibiting layer.
The surfaces of metal workpieces to be painted have to be cleaned and degreased before painting in order to establish the conditions required for durable painting, i.e. for firm paint adhesion to the workpiece surface. To this end, the surfaces of the workpiece to be painted are treated with a solution containing suitable degreasing agents which is either sprayed onto the surfaces to be painted or which serves as a dip bath for the workpiece. After this degreasing step followed by a rinse, the workpieces are phosphated to improve their corrosion resistance. Through this phosphating step, the workpiece surface is oxidatively attacked, resulting in slight erosion of a surface layer. The roughening effect this has on the surface improves the adhesion of the paint subsequently applied. In order to terminate the oxidative surface reaction, the workpiece surface is passivated in another treatment step; thus, overall, the pretreatment of the workpiece is very complicated.
In order to protect soft iron surfaces against the corrosive effect of salt water, it is known (WO 96/27696) that comparatively small amounts of polyaspartic acid can be added to the salt water. The corrosion-inhibiting effect of polyaspartic acid and polyaspartic polymers of polyaspartic acid and its salt or amide is also used in coolants and lubricants for the machining of workpieces (WO 95/10583, WO 95/24456). However, these corrosion-inhibiting additions to liquids with a corrosive effect on metal surfaces do not impart any teaching as to how surfaces of metal workpieces should be treated in readiness for durable painting.
Finally, it is known (WO 95/24456) that an aqueous solution of a polyamino acid of which the amino acid group may contain inter alia aspartic acid can be used to clean a metal surface. Although the metal surface treated with such a solution can be kept from corroding by maintaining a comparatively low pH value of at most 7, the protection against corrosion required for subsequent painting is not achieved in this way.
Accordingly, the problem addressed by the present invention was to develop a process for the pretreatment of a metal workpiece for painting of the type mentioned at the beginning in such a way that the number of process steps involved would be considerably reduced without impairing paint adhesion or endangering corrosion prevention.
The present invention relates to a process for pretreating a metal workpiece for painting, the surface to be painted being cleaned with a solution containing degreasing agents and being coated with a corrosion-inhibiting layer, characterized in that the surface of the workpiece to be painted is treated with a solution of polyaspartic acid for application of a corrosion-inhibiting layer.
The solution containing a degreasing agent is preferably an aqueous solution of a so-called neutral cleaner or an alkaline cleaner. Neutral cleaners generally have a pH value in the range from about 7 to about 9 and contain nonionic surfactants as degreasing agents. Alkaline cleaners are adjusted to pH values above 8.5 by addition of alkalis or alkaline builders. They preferably contain anionic surfactants as degreasing agents.
Any reference in the following to a solution of polyaspartic acid or a copolymer containing aspartic acid is meant to convey that the solution contains the free acid and/or acid anions. It is well-known that the ratio of free acid to acid anions is dependent on the pH value of the solution. The aspartic acid units in polyaspartic acid can be joined together by so-called -linkage or so-called -linkage. In general, both linkage types are present alongside one another in a polymer strand. The ratio of - to -linkages is of no relevance to the present invention. A copolymer containing aspartic acid is a copolymer which contains other monomeric groups besides aspartic acid units. These other monomeric groups may be other amino acids, but also other polymerizable carboxylic acids. Copolymers of which at least 50 mole % and preferably at least 80 mole % consists of aspartic acid are preferably used.
Any brief reference to polyaspartic acid in the following is intended to encompass both polyaspartic acid itself and a copolymer containing aspartic acid.
Through the treatment of the surface of the workpiece to be painted with a solution of polyaspartic acid, the workpiece surface can be covered with a thin organic film of an organic polymer of which the branched structure ensures not only a firm connection with the substrate, but also favorable adhesion promotion of the paint. Accordingly, the use of a polyaspartic acid for pretreating a metal workpiece to be painted, which can be treated with the polyaspartic acid solution in the usual way by dipping or spraying, affords the advantage that, by comparison with phosphating, a treatment step corresponding to passivation is saved so that the risk of corrosion posed by inadequate passivation can be eliminated.
The metal surface coated with a layer of polyaspartic acid may be directly painted. By this is meant that rinsing steps at most, but no other operations, such as for example transport of the workpieces to another processing station, forming or assembly steps, take place between the treatment with the solution containing aspartic acid and painting. However, the described treatment with the solution containing polyaspartic acid may also be used as a temporary corrosion prevention measure. In other words, it provides the metal surface with sufficient protection against corrosion, even without direct painting, for the metal workpieces to be transported, formed or assembled before painting. If such processes do take place, aftertreatment with another solution for strengthening or lengthening the corrosion-inhibiting effect obtained in step b, immediately before the final painting process is preferred. This aftertreatment may be, for example, phosphating, treatment with a solution of chromium compounds, a solution of complex fluorides of boron, silicon, titanium and/or zirconium and/or a solution or dispersion of other organic polymers. The workpiece may be cleaned once more before this aftertreatment.
Particularly favorable conditions in this regard are established if, as in another embodiment of the invention, the surface of the workpiece to be paintedxe2x80x94for simultaneous degreasing and coatingxe2x80x94is treated with a solution of polyaspartic acid or a polymer containing aspartic acid and a degreasing agent. In this case, a single treatment step can provide both for degreasing and for corresponding corrosion prevention. The solution preferably contains a degreasing agent based on the sodium salt of iminodisuccinic acid. The polyaspartic acid acts as a dispersant for the fatty particles detached from the workpiece surface by the sodium salt of iminodisuccinic, so that the detached fatty particles are kept in dispersion by this dispersant and are prevented from resettling on the workpiece surface. However, the polyaspartic acid not only has a dispersing effect on the fatty particles, it also forms a corrosion-inhibiting surface layer which is of particular advantage to adhesion promotion and which establishes favorable conditions for durable painting, especially since the sodium salt of iminodisuccinic acid provides for corresponding roughening of the workpiece surface.
The ratio by weight of polyaspartic acid to the sodium salt of iminodisuccinic acid should be between 10:1 and 1:10, a minimum polyaspartic acid concentration of 2% having to be established in the solution to obtain the desired effects. Corresponding additives, for example wetting agents or the like, may of course be mixed with the sodium salt of iminodisuccinic acid.
If the solution of the iminodisuccinic acid sodium salt and the polyaspartic acid laden with the fats detached from the workpiece surface is subjected to membrane filtration, the detached fats and impurities can be separated and the solution of the iminodisuccinic acid sodium salt and the polyaspartic acid may be recycled for treatment of the workpiece. This is because the solution of the organic degreasing agent and the polyaspartic acid permeate the membrane filter as permeate while the fats and impurities can be separated as retentate and disposed of. The metal ions dissolved from the workpiece surface by the iminodisuccinic acid sodium salt may then be separated from the permeate to prevent these metal ions from accumulating.
The corrosion prevention achieved where degreasing is carried out with the sodium salt of iminodisuccinic acid may if necessary be increased by subjecting the surface of the workpiece to be painted to another treatmentxe2x80x94after degreasingxe2x80x94in a 10 to 40% solution of the polyaspartic acid in order to improve coating with the organic polymer formed by the polyaspartic acid. The solution accumulating from this aftertreatment may of course also be subjected to membrane filtration to enable the polyaspartic acid to be reused for treatment of the workpiece after the removal of unwanted impurities.